9. Spectroscopy - Spectroscopy Spectroscopy Our goals for...

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Spectroscopy Spectroscopy Our goals for this section: Describe the characteristics of continuous, emission, and absorption spectra Explain Kirchhoff’s laws for the generation of different types of spectra Review Bohr’s theory of the atom Explain how electronic transitions in atoms produce the unique emission and absorption spectra of those atoms Describe the general features of spectra produced by molecules Summarize the huge variety of information which can be derived from spectra
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If all bodies were black-bodies, it would be hard to learn much about them from a distance How would you calculate the Doppler shift of a body if it only emitted a continuous black-body spectrum? As we have discussed, composition is not an important factor in determining whether a body emits as a black-body Luckily, most objects don’t emit exactly as black-bodies Spectroscopy is the science of examining the spectrum of particular objects or substances to determine their physical characteristics (chemical composition, kinematics, etc.) Types of Spectra
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If we use a spectroscope to examine the light emitted by a standard incandescent bulb, we would find that it emits radiation at all wavelengths, forming a continuous spectrum—a good approximation of a black-body A spectroscope is a device which uses a prism or other means to split a beam of light into its component colors A simple spectroscope might look like this:
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A continuous spectrum is one in which there is emission at all frequencies with no exceptions A black-body emits a continuous spectrum A continuous, flat spectrum over the visible part of the electromagnetic spectrum frequency intensity
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frequency intensity
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frequency intensity
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frequency intensity
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There are two general types of discontinuous spectra For example, a diffuse, hot gas cloud emits light at only a few discrete frequencies spectrum of diffuse hydrogen gas
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We call light emitted at these discrete frequencies emission lines We can adjust the intensity of these lines by changing the amount of hydrogen or by increasing its temperature , but the lines will always occur at the same frequencies, regardless of temperature These lines form a “fingerprint” for hydrogen--other atoms and molecules produce their own unique set of emission lines If we take the spectrum of the Sun, we find the reverse phenomenon The spectrum of the sun looks like a continuous spectrum which is interrupted by dark gaps or absorption lines
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the visible-light portion of the solar spectrum The absorption lines in the Sun’s spectrum are referred to collectively as Fraunhofer lines, after 19th c. physicist Joseph Fraunhofer, who cataloged more than 600 of them
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Absorption lines can be produced in the laboratory by passing a beam of light from a continuous source through a cool gas : If hydrogen gas is used as the cool gas, then the absorption lines appear at precisely the same wavelengths as
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This note was uploaded on 03/01/2011 for the course ASTRO 1a03 taught by Professor Samantha during the Spring '11 term at McMaster University.

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9. Spectroscopy - Spectroscopy Spectroscopy Our goals for...

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